1. Field of the Invention
The present invention relates to a method and apparatus for providing a combustibly inert air or gas, for use to inert fuel tanks and/or to suppress fire, or for any other use of a combustibly inert air or gas.
2. Description of the Prior Art
Advancements in enemy weapon technology against aircraft for the past several years has significantly increased the vulnerability of all types of aircraft conducting combat missions. Mission analysis studies conducted on air to surface (ATS) type combat aircraft indicate 90% of a strike force could be lost in each combat mission, without advanced survivability features, which could equate to billions of dollars loss in aircraft for a limited war period. Advanced aircraft survivability features being studied include: (1) Reduced radar cross section, (2) Electronic counter measures (ECM), and (3) Fuel tank protection.
The fuel tanks represent the largest vulnerable areas on all fixed wing aircraft and fuel tank inerting, which provides a non explosive atmosphere, significantly reduces the aircraft vulnerable area with an associated increase in survivability and a reduction in life cycle cost relative to an aircraft without inerting.
Aircraft fuel tanks are vulnerable to both natural and weapons threats. Natural threats include lightning strike, static discharges during refueling, and onboard ignition sources including engine fragments. Weapons threats include air-to-air, ground-to-air, and air-to-ground weapon fragments, etc.
One way of suppressing fire and explosions is to provide an inert environment within the fuel tank. A method of achieving this is to fill the tank ullage space with an inert gas, such as nitrogen, so as to maintain the oxygen concentration at or below nine percent by volume.
Full time gaseous inerting of fuel tanks is presently being accomplished with onboard stored liquid nitrogen. However, the use of liquid nitrogen is not always desirable because of weight, logistic, safety, and cost factors. Cost and weight penalties associated with the use of liquid nitrogen are especially high for large aircraft.
Major emphasis is being placed on developing onboard nitrogen generating systems to eliminate the logistics problem associated with providing liquid nitrogen. Hollow fiber permeable membrane and physical sorption inert gas generators (IGG) have emerged as prime candidates for onboard nitrogen generation. Unfortunately, the weight associated with these concepts is highly dependent on ullage volume and aircraft descent rate. Studies indicate that design inertant flow rates result from aircraft descent operation. Descent flow rates results from ullage volume repressurization required to preclude exceeding fuel tank pressure limitations when descending from high altitude to low altitude.
The most promising technology concepts for onboard generation of inert gas (IGG) from engine bleed air are the hollow fiber and molecular sieve designs under development by AiResearch Corporation of California and the Bendix Corporation respectively. Each of these concepts results in unacceptable weight and volume if the IGG units are sized to provide insert gas (9% oxygen concentration) flow rates required for larger aircraft during descent such as a strategic penetrator.
An onboard nitrogen concept would weight approximately 1800 lbs if sized to meet the requirements of the strategic penetrator. This weight penalty, and the associated concept, are considered unacceptable for aircraft having large fuel volume.
The novel system of our invention utilizes a high pressure stored gas subsystem for providing inert gas descent flow rates and a small IGG component to satisfy all fuel scrub and climb and cruise inertant requirements.
Prior art inert gas systems existing in the patent literature are disclosed by the following U.S. Pat. Nos. 2,365,624, granted Dec. 19, 1944, to Gaetano Cantello and Dominic Cantello; 2,944,987 granted July 12, 1960, to Donald A. Potter and Kurk Staiger; 2,952,428, granted Sept. 13, 1960, to Oscar C. Bridgeman; 2,983,405, granted May 9, 1961, to Colin A. M. Tayler; 3,389,829, granted June 25, 1968 to Alfred E. Stanford; 3,389,972, granted June 25, 1968, to John E. Pottharst, Jr.; 3,464,801, granted Sept. 2, 1969, to William F. Barston; 3,467,349, granted Sept. 16, 1969, to Robert A. Gautier; 3,628,758, granted Dec. 21, 1971 to Richard A. Nichols; 3,691,730, granted Sept. 19, 1972, to William G. Hickey and Richard L. Kenyon; 3,693,915, granted Sept. 26, 1972, to Jack M. Ulanovsky; 2,756,215, granted July 24, 1956 to Glenn A. Burgess, George E. Hlavka, and Leighton S. King; 3,788,039, granted Jan. 29, 1974, to Kenneth R. Bragg; 3,847,298, granted Nov. 12, 1974, to Mackenzie L. Hamilton; and 3,948,626, granted Apr. 6, 1976, to Kenneth R. Bragg. These patents and the article entitled "Bang Go the Big Ones", appearing in the Apr. 12, 1980, issue of The Economist, pages 52 and 53, should be carefully considered for the purpose of putting the present invention into proper perspective with respect to the prior art.